Synesthesia is involuntary, consistent, and lifelong. A synesthete does not imagine that Tuesday is green or that a trumpet is scarlet; the pairing simply arrives with the stimulus, the same way every time, and has usually done so since childhood.
Large screening studies estimate that 2 to 4 percent of people have at least one form. Dozens of forms are documented, named by their trigger and their result: grapheme-color, sound-to-color, lexical-gustatory. The trigger is called the inducer and the resulting experience the concurrent.
Researchers verify synesthesia with consistency testing, since genuine pairings hold steady over months and years while invented ones drift. It runs in families, is not a disorder, and most synesthetes describe it as neutral or pleasant. The word joins the Greek syn (together) and aisthesis (sensation).